The present invention relates to a polishing machine, more precisely relates to a polishing machine capable of polishing both sides (faces) of work pieces.
In a conventional polishing machine, an external gear and an internal gear are rotated at different angular velocity, so that a carrier revolves and moves round along an orbit as a planet gear. An upper polishing plate and a lower polishing plate, which are respectively provided on an upper and a lower sides of the carrier, pinch and polish work pieces, which are held in the carrier. This structure has been employed not only in polishing machines but also lapping machines. By employing this structure, both sides of the work pieces can be highly precisely polished in a short time. So the machines are properly used for polishing thin work pieces, e.g., silicon wafers for semiconductor chips.
The conventional polishing machine will be explained with reference to FIG. 10.
Polishing cloths are provided on surfaces of an upper polishing plate 112 and a lower polishing plate 114 to form polishing faces. An external gear 116 and an internal gear 118 are provided. Each carrier 120 has through-holes 121, in which work pieces 121 are respectively accommodated. The carriers 120 are engaged with the internal gear 118 and the external gear 116 as a planet gear, so that the carrier 120 are rotated.
The upper polishing plate 112 is connected to a rotary head 112a. A gear 112c is fixed to a lower end of a shaft 112b, which is downwardly extended from the rotary head 112. The gear 112c is engaged with a gear 112d; the gear 112d is engaged with a gear 112e. The gear 112e is coaxially fixed to a spindle 126, so that the gear 112e is rotated together with the spindle 126. A gear 114a, which is coaxially provided to the lower polishing plate 114, is engaged with a gear 114b, which is coaxially fixed to the spindle 126. The internal gear 118 is linked with a gear 118b, which is coaxially fixed to the spindle 126, by a gear 118a, which is coaxially provided to the internal gear 118. With this structure, the external gear 116, internal gear 118 and the polishing plates 112 and 114 are rotated by a four-way system including one driving unit.
The spindle 126 is connected to an adjustable reduction gear unit 132. The adjustable reduction gear unit 132 is connected to a motor 134 by a belt 136, so that rotational speed of the spindle 126 can be adjusted.
In the conventional polishing machine, gear ratio between the gears 116a and 116b and gear ratio between the gears 118a and 118b are defined, for example, to make angular velocity of the internal gear 118 faster than that of the external gear 116. In this case, the carrier 112, which engages with the external gear 116 and the internal gear 118, moves round in the same direction as a rotational direction of the internal gear 118, e.g., the counterclockwise direction, and revolves in the clockwise direction. The lower polishing plate 114 rotates in the counterclockwise direction; the upper polishing plate 112 rotates in the clockwise direction due to the gear 112d.
Note that, the rotational direction, rotational speed, etc. of the carriers 120 may be adjusted by changing the angular velocity of the external gear 116 and the internal gear 118 according to polishing conditions.
To polish both sides (surfaces) of the work pieces 121, a liquid abrasive including polishing grains is supplied to the both surfaces to be polished, so that the both surfaces of the works 121 can be properly polished. In the case of polishing silicon wafers, an alkali liquid abrasive (slurry) is supplied to the surfaces of the silicon wafers.
The liquid abrasive is supplied to the work pieces through vertical through-holes of the upper polishing plate 112. The liquid abrasive is usually fallen onto the work pieces by a pump and the gravitational force. The liquid abrasive, which has been fallen from the through-holes, is supplied to the polishing face of the upper polishing plate 112 and upper faces of the work pieces 121. And, the liquid abrasive is further supplied to the polishing face of the lower polishing plate 114 and lower faces of the work pieces 121 via spaces between the adjacent carriers 120.
FIG. 11 is a plan view showing an arrangement of the carriers 120 in the polishing machine shown in FIG. 10. There are the spaces "A" between the adjacent carriers 120. The spaces "A" are formed in an inner part and an outer part, and they have enough area so that the liquid abrasive is properly supplied onto the upper face of the lower polishing plate 114. As described above, the liquid abrasive for polishing the both faces of the works 121 can be supplied, by a simple supplying means, from upper side.
In the conventional polishing machine, the liquid abrasive can be properly supplied, and complex movement of the carriers 120 can be executed, so the work pieces 121, e.g., silicon wafers, can be uniformly polished. Thus, the flatness of the polished work pieces can be improved. By simultaneously polishing the both faces of the work pieces 121, polishing efficiency can be increased.
However, in the conventional polishing machine, the carriers 120 move between the external gear 116 and the internal gear 118, so size of work pieces is limited. These days, silicon wafers having greater diameter are required, but the conventional polishing machine cannot be employed to polish the large silicon wafers. Namely, it is impossible to use large carriers, whose diameters are greater than radius of the polishing plates. And, the polishing faces of the polishing plates cannot be used efficiently.
Further, a complex gear mechanism is assembled in the conventional polishing machine, so it is very difficult to make the size of the machine bigger and manufacturing cost must be higher.